On the first 4 exams of his math class, William got an average score of 78. What does he need on the next exam to have an overall average of 80?
Explanation: Let his score on the next exam be $x$ The sum of all of his scores is then $4 \cdot 78 + x$ The same sum must also be equal to $5 \cdot 80$ Solve: $x = 5 \cdot 80 - 4 \cdot 78 = 88$.